1. Field of the Invention:
The present invention relates to spacecraft propulsion. More specifically, the present invention relates to electrostatic ion thrusters for use on a spinning spacecraft.
While the present invention is described herein with reference to a particular embodiment for an illustrative application, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teaching provided herein will recognize additional modifications, applications and embodiments within the scope thereof.
2. Description of the Related Art:
Although chemical thrusters are widely used for the propulsion of planetary or orbital spacecraft, an alternative technique of electrostatic ion propulsion is well known in the art. U.S. Pat. No. 3,052,088 to Davis et al and U.S. Pat. No. 3,156,090 to Kaufman describe typical electrostatic ion propulsion devices. Electrostatic ion thrusters have a high specific impulse provide equivalent thrust with generally less propellant than that required by chemical thrusters. This results in generally longer maneuver life. Since maneuver life is a significant limiting factor on the mission life of a satellite, it would be advantageous to employ electrostatic ion thrusters in orbital satellites.
It is known, however, that the use of an electrostatic ion thruster for station keeping on the spinning portion of a spin-stabilized spacecraft requires some thrust modulation to maintain spacecraft attitude. That is, when the center of mass of the system is offset relative to the spin axis, the ion thruster will produce a torque about the center of mass. The thrust may be modulated over a specified portion of the spacecraft revolution, to produce an offsetting torque sufficient to maintain a desired spacecraft attitude. This thrust modulation eliminates the need to use chemical thrusters to offset the undesirable torque and maintains the advantage of the ion thruster (viz. high specific impulse) while mitigating the risk of chemical-exhaust contamination.
In addition, modulated ion propulsion offers significant performance, lifetime and cost advantages over chemical thrusters. The ability to realize these advantages is dependent on the ability to modulate thrust without reducing the lifetime of the ion propulsion system or its compatibility with the spacecraft.
One prior ion thrust modulation technique achieved thrust modulation by modulating the output voltages of screen and accelerator power supplies used to form an exhaust beam. With this technique, ion beam current is essentially maintained constant while the energy transferred to the ions from the screen electrode is reduced. This reduction in energy results in a slowing of the ions which causes a defocusing of the ion beamlets path. The defocusing of the ion beamlets results in increased ion impingement on the electrodes. Increased ion impingement coupled with the increased negative voltage of the accelerator electrode during modulation contribute to an increased electrode wear rate, an additional limiting factor on spacecraft mission life.
An additional disadvantage of the sputtering caused by the slowing of the ions is that the sputtered material may contaminate sensitive spacecraft surfaces.
Further, during operation of an ion thruster, it is often desirable to modulate down to 60 percent or less of the nominal thrust. At such low levels of thrust, the impingement of ions onto the electrodes by the modulated ion thrusters of the prior art may be so great that the wear on the electrodes is magnified many times. This problem could be partially offset by modulating to a higher percentage of the nominal thrust. However, this would require the period of modulation to be extended in order to provide the desired offsetting torque. As a result, the thruster must be activated for a longer period of time in order to complete the desired station keeping maneuver. This longer duration of duty cycle also reduces mission life.
In addition, conventional systems typically require a three-grid ion extraction assembly in order to achieve sufficient thrust modulation. Such an assembly is typically heavy, costly and difficult to assemble.
There is therefore a recognized need in the art for a lightweight, low cost, long life ion thruster which offers thrust modulation without a reduction in the exit velocity of propellant ions.